The present invention relates to apparatus and methods for monitoring the level of a constituent in the blood of a living organism.
Certain constituents in the blood affect the absorption of light at various wavelengths by the blood. For example, oxygen in the blood binds to hemoglobin to form oxyhemoglobin. Oxyhemoglobin absorbs light more strongly in the infrared region than in the red region, whereas hemoglobin exhibits the reverse behavior. Therefore, highly oxygenated blood with a high concentration of oxyhemoglobin and a low concentration of hemoglobin will tend to have a high ratio of optical transmissivity in the red region to optical transmissivity in the infrared region. The ratio of transmissivities of the blood at red and infrared wavelengths can be employed as a measure of oxygen saturation.
This principle has been used heretofore in oximeters for monitoring oxygen saturation of the blood in the body of a living organism as, for example, in patients undergoing surgery. As disclosed in U.S. Pat. No. 4,407,290, oximeters for this purpose may include red light and infrared light emitting diodes together with a photodetector. The diodes and photodetector typically are incorporated in a probe arranged to fit on a body structure such as an earlobe or a fingertip, so that light from the diodes is transmitted through the body structure to the photodetector. The infrared and red light emitting diodes are switched on and off in alternating sequence at a switching frequency far greater than the pulse frequency. The signal produced by the photodetector includes alternating portions representing red and infrared light passing through the body structure. These alternating portions are segregated by sampling devices operating in synchronism with the red/infrared switching, so as to provide separate signals on separate channels representing the red and infrared light transmission of the body structure. After amplification and low-pass filtering to remove signal components at or above the switching frequency, each of the separate signals represents a plot of optical transmissivity of the body structure at a particular wavelength versus time.
Because the volume of blood in the body structure varies with the pulsatile flow of blood in the body, each such signal includes an AC component caused only by optical absorption by the blood and varying at the pulse frequency or heart rate of the organism. Each such signal also includes an invariant or DC component related to other absorption, such as absorption by tissues other than blood in the body structure. According to well known mathematical formulae, set forth in said U.S. Pat. No. 4,407,290, the oxygen saturation in the blood can be derived from the magnitudes of the AC and DC components of these signals.
As also set forth in the '290 patent, the same general arrangement can be employed to monitor constituents of the blood other than oxygen such as carbon dioxide, carbon monoxide (as carboxyhemoglobin) and/or blood glucose, provided that the other constituents have some effect on the optical properties of the blood.
Measurement apparatus and methods of this type have been widely adopted in the medical profession. However, the signal sampling devices must be precisely synchronized with the switching devices used to provide the alternating rods and infrared illumination. The circuitry required to maintain this synchronization adds cost and complexity to the apparatus. Moreover, the signals representing light transmission at each wavelength are necessarily discontinuous.
Moreover, such apparatus and methods have been subject to interference from ambient light falling on the photodetector. The devices used to recover the meaningful signal components after amplification of the photodetector signal have been provided with circuits for canceling components caused by ambient light. Generally, these circuits operate by obtaining a "dark current" signal representing the amplified photodetector signal during intervals when both of the light emitting diodes are off and hence all of the light falling on the photodetector represents ambient light. The dark current signal value can be used to cancel the ambient light component in signals representing infrared and red light.
This approach provides only a partial solution to the ambient light interference problem. The dark current cancellation circuitry adds complexity and cost to the apparatus. Also, the ambient light signals may saturate or overload the initial or front end amplifier connected to the photodetector, resulting in unpredictable fluctuations in the amplifier output. To prevent saturation of the front end amplifier, its gain may be limited, but this in turn requires higher gain in subsequent stages, more amplification stages or both. Baffles can be used to limit ambient light reaching the photodetector, but these add further complexity and cost, and are only partially effective.
Electromagnetic interference capacitively or inductively coupled to the photodetector and/or leads can also saturate the front end amplifier or create spurious signals. The shielding used to protect the photodetector and leads from this interference adds further cost, complexity and bulk.
A new solution to the problems of electromagnetic and ambient light interference is set forth in the co-pending, commonly assigned United States patent application of Alan Dean Martin entitled "Blood Parameter Monitoring Apparatus and Methods", filed on the same day as the present application. The disclosure of said application of Martin is incorporated by reference herein. As disclosed in said application of Martin, the light emitted by the illuminating means such as a light emitting diode is varied at one or more carrier frequencies. Therefore, the photodetector signal will include one or more components at the carrier frequency or frequencies, and these components will represent the light transmitted through the patient's body structure from the light emitting means. Modification means are provided for modifying the photodetector signal, preferably prior to any amplification, so as to increase the ratio of the carrier frequency component or components to other components of the signal. Typically, the modification means include a filter such as a passive resonant circuit, resonant at the carrier frequency or frequencies employed. The resonant circuit is arranged to pass only signal components at the carrier frequency or frequencies, while substantially attenuating other components. The modification means effectively eliminates both components of the signals due to ambient light, and also effectively eliminates typical electromagnetic interference signals. Thus, the front end amplifier cannot be overloaded by these spurious signal components. Also, because the ambient light components are effectively eliminated by the modification means, the device need not incorporate separate "dark current" compensation circuitry.
The preferred apparatus set forth in the aforementioned Martin application, however, utilizes a time division multiplexing scheme. Thus, light of different wavelengths is applied in a sequence of alternating bursts at a predetermined switching frequency, with the light within each burst varying in amplitude at the carrier frequency or frequencies. The modified photodetector signal from the modification means or resonant circuit is sampled at predetermined times corresponding to the alternating bursts of light at the different wavelengths, so that the sampling procedure effectively separates signals representing transmissivity at each wavelength. This arrangement thus requires switching, sampling and timing circuitry.
Accordingly, there have been significant needs heretofore for still further improvements in blood constituent monitoring apparatus such as medical oximeters.